The thermal print head is an important part of the heat printer and used in thermal printer, thermal transfer printer and other heat printer. The thermal print head is provided with heating element units, and corresponding graphs or characters can be printed by controlling whether each unit generates heat. Particularly, the thermal printer, making use of the heating element of the print head generating heat, enables a chemical change of a thermo-sensitive layer on the surface of the printing paper, so as to develop colors and form characters or images, while the thermal transfer printer, making use of the heating element of the print head generating heat, heats up and melts the pigment matters on the carbon ribbon onto the surface of the printing paper to form characters or images.
Take the commonest thermal printer as an example. The heating element units of the thermal print head used are heatable square dots, and one thermal print head has 320 heatable square dots each occupying an area of 0.25 mm*0.25 mm. In printing, corresponding heatable square dots in the dot matrix are controlled to generate heat according to the characters and images to be printed, and then the contents to be printed may be printed on any place of the thermal printing paper. The aforesaid heating element units are represented as resistance in electrical characteristic.
Obviously, normal printing will be affected if any heating element unit of the thermal print head cannot normally generate heat when a failure such as open circuit occurs. Generally, however, when an individual heating element unit of the thermal print head fails to normally generate heat, the influence to printing cannot be instantly recognized by observing the printing effect, while such failure might have significant influence in some printing occasions. For instance, the heat printer is usually used to print the bar code. The bar code which is compiled with bars and gaps having different widths and different reflectivities according to a certain encoding rule can express information such as a group of numbers or letter symbols, and is used in various occasions such as commodity and printed work identification. When the heat printer is used for printing, according to requirements of bar code encoding, corresponding bars and gaps are formed on the printing medium by controlling whether each heating element unit of the thermal print head generates heat. If a certain heating element unit of the thermal print head is broken or damaged, this heating element unit cannot generate heat after it is galvanized, resulting in inability of color development of the dot corresponding to this heating element unit. Thus, when the bar code is printed, the arranging rule of the bars and gaps of the bar code is changed by the blank bar generated at the place corresponding to the broken heating element, which leads to a data error. As the data information represented by the bar code is very important, and the above error occurring in printing is hard to be detected, measures therefore are necessarily to be employed to avoid the error.
Owing to the above reason, the heat printer usually detects the heating element units of the thermal print head. A traditional method is to regularly remove the print head from the printer and to detect the resistance value of the heating element units of the thermal print head using an exclusive detecting circuit. As the periodic detection cannot timely reflect the on or off state of the heating element unit of the print head, and the print head should be removed, and the operation is complicated, therefore, practical requirements cannot be satisfied.
Japanese Patent publication No. JP2007-268918 discloses a thermal print head detecting device. FIG. 1 is a circuit principle diagram of the thermal print head detecting device provided in this Japanese Patent. A control device of the thermal print head detecting device includes a normal printing power supply and an open circuit detecting power supply 21. When the printer receives a printing instruction, a power supply shifting switch 22 turns on the normal printing power supply; and when the printer receives a detecting instruction, the power supply shifting switch 22 turns on the open circuit detecting power supply 21. A print head heating element unit HE is connected in parallel with a resistor 25, with the parallel end being connected to an output end of the power supply shifting switch 22 through a resistor 23 and the other end being grounded. A detecting circuit is constructed with the turned-on open circuit detecting power supply 21, the resistor 23, the heating element unit HE and the detecting resistor 25. In detection, the heating element unit HE can be judged whether it is broken by detecting a voltage of the resistor 23. Thus, the on or off state of the heating element unit of the print head can be detected on line by shifting between the normal printing power supply and the open circuit detecting power supply.
The shortcomings of the prior art above lie in: the normal printing power supply and the print head detecting power supply should be separately provided and the power supply switch should be provided for this two power supplies, which make the circuit complex. Moreover, if a too small resistance value is set for the resistor 23 and the resistance value of the heating element unit is imprecisely detected, the existing failure may be hardly discovered; and if a too high resistance value is set for the resistor 23, the resistor 23 will consume too much power in normal printing, and the printing effect will be affected.